The present invention generally relates to high solids low VOC (volatile organic component) coating compositions and more particularly to low VOC clear coating compositions suited for multi-layered coatings used in automotive OEM and refinish applications.
Basecoat-clearcoat systems have found wide acceptance in the past decade as automotive finishes. Continuing effort has been directed to such coating systems to improve the overall appearance, the clarity of the topcoat, and the resistance to deterioration. Further effort has been directed to the development of coating compositions having low volatile organic content (VOC). A continuing need exists for clear coating formulations which provide an outstanding balance of performance characteristics after application, particularly solvent and mar and etch-resistance.
Examples of compositions used to form a protective coatings include those disclosed in U.S. Pat. No. 4,533,716 (Okoshi et al.). These compositions are prepared using a triazine-type resin formed by a co-condensation reaction with formaldehyde, a polyol, and as required a monohydric alcohol. The polyols used in the condensation reaction may be prepared by an esterification reaction between higher fatty acids and epoxy compounds having a number average molecular weight of 174 to 4,000. The polyols used in the U.S. Pat. No. 4,533,716 invention do not include epoxy compounds. The U.S. Pat. No. 4,533,716 coating compositions are disclosed to include a triazine-type resin and a polyisocianate.
The present invention is directed to a clear coating composition comprising isocyanate, melamine, and epoxy components wherein the isocyanate component comprises an aliphatic polyisocyanate having on an average 2 to 6 isocyanate functionalities.
The present invention is also directed to a method of producing a clear coating on a substrate comprising:
applying a layer of a clear coating composition comprising isocyanate melamine, and epoxy components wherein the isocyanate component comprises an aliphatic polyisocyanate having on an average 2 to 6 isocyanate functionalities, and curing the layer into the clear coating.
One of the advantages of the present invention is its low VOC, significantly below the current guidelines of Environment Protection Agency (EPA) of the United States.
Another advantage is etch and mar resistance of the coating resulting from the coating composition of the present invention.
Yet another advantage is the clarity of the coating resulting from the coating composition of the present invention.
As used herein:
“Two-pack coating composition” means a thermoset coating composition comprising two components stored in separate containers. These containers are typically sealed to increase the shelf life of the components of the coating composition. The components are mixed prior to use to form a pot mix. The pot mix has a limited potlife typically of minutes (5 minutes to 45 minutes) to a few hours (4 hours to 6 hours). The pot mix is applied as a layer of desired thickness on a substrate surface, such as an autobody. After application, the layer is cured under ambient conditions or cure-baked at elevated temperatures to form a coating on the substrate surface having desired coating properties, such as high gloss, mar-resistance and resistance to environmental etching.
“Low VOC coating composition” means a coating composition that includes less then 0.48 kilograms of organic solvent per liter (4 pounds per gallon) of the composition, as determined under the procedure provided in ASTM D3960.
“High solids composition” means a coating composition having solid component in the range of from 65 to 100 percent and preferably greater than 70 percent, all in weight percentages based on the total weight of the composition.
“Clear coating composition” means a clear coating composition that produces upon cure, a clear coating having DOI (distinctness of image) and 20° gloss of more than 70.
“Polymer solids” or “Composition solids” means a polymer or composition in its dry state.
“Aliphatic” as employed herein includes aliphatic and cycloaliphatic materials.
“Crosslinkable” means that the individual components of the adduct contain functionality which react within the composition of the invention to give a coating of good appearance, durability, hardness and mar resistance.
Applicants have unexpectedly discovered that contrary to conventional approaches used in typical thermoset coating compositions, i.e., those involving a film forming polymer and crosslinking component, a very viable route lies in a combination of what would traditionally be considered crosslinking agents for producing a unique low VOC high solids clear coating composition which provides coatings with superior properties, such as mar and etch resistance.
The clear coating composition includes isocyanate, melamine, and epoxy components. The isocyanate component includes an aliphatic polyisocyanate having on an average 2 to 6, preferably 2.5 to 6 and more preferably 3 to 4 isocyanate functionalities. The coating composition includes in the range of from 35 percent to 70 percent, preferably in the range of from 40 percent to 65 percent and more preferably in the range of from 45 percent to 60 percent of the aliphatic polyisocyanate, the percentages being in weight percentages based on the total weight of composition solids.
Examples of suitable aliphatic polyisocyanates include aliphatic or cycloaliphatic di-, tri- or tetra-isocyanates, which may or may not be ethylenically unsaturated, such as 1,2-propylene diisocyanate, trimethylene diisocyanate, tetramethylene diisocyanate, 2,3-butylene diisocyanate, hexamethylene diisocyanate, octamethylene diisocyanate, 2,2,4-trimethyl hexamethylene diisocyanate, dodecamethylene diisocyanate, 1,3-cyclopentane diisocyanate, 1,2-cyclohexane diisocyanate, 1,4cyclohexane diisocyanate, isophorone diisocyanate, 4-methyl-1,3-diisocyanatocyclohexane, trans-vinylidene diisocyanate, dicyclohexylmethane-4,4′-diisocyanate, 3,3′-dimethyl-dicyclohexylmethane4,4′-diisocyanate, meta-tetramethylxylylene diisocyanate, polyisocyanates having isocyanaurate structural units such as the isocyanurate of hexamethylene diisocyanate and isocyanurate of isophorone diisocyanate, the adduct of 2 moles of a diisocyanate, such as hexamethylene diisocyanate, uretidiones of hexamethylene diisocyanate, uretidiones of isophorone diisocyanate or isophorone diisocyanate, and one mole of a diol such as ethylene glycol, the adduct of 3 moles of hexamethylene diisocyanate and 1 mole of water (available under the trademark Desmodur N of Bayer Corporation, Pittsburgh, Pa. or Tolonate HDT-LV from Rhodia Co., Cranbury, N.J.). If desired, the isocyanate functionalities of the polymeric isocyanate may be capped with a monomeric alcohol, isopropanol or isobutanol to prevent premature crosslinking in a one-pack composition. Some suitable monomeric alcohols include methanol, ethanol, propanol, butanol and hexanol.
Some other suitable blockers include lactams, oximes, malonic esters, alkylacetoacetates, triazoles, pyrazoles (e.g. dimethyl pyrazole), phenols and amines. Compositions of the present invention include one or more melamine. A melamine used in the present invention includes, in part, alkylated melamine-formaldehyde resin. The alkylated melamine formaldehyde resin maybe partially or fully alkylated. As examples of partially alkylated melamines, Cymel 1158 Cymel 324, Cymen 327 (from Cytec Ind., from West Paterson, N.J.), Resimine BM 9539 (from Solutia, Inc., St. Louis, Mo.) can be mentioned. As examples of fully aklykated melamines, Cymel 350, and Cymel 301 (from Cytec Co., from Cytec. Ind., West Paterson, N.J.) and Resimine 764 (from Solutia Inc., St. Louis, Mo.) can be mentioned. A composition of the present invention may comprise from about 10 weight percent to about 40 weight percent of a melamine, preferably from about 15 weight percent to 35 weight percent of a melamine.
Compositions of the present invention include one or more oligomeric epoxy compounds, preferably containing at least two epoxy functionalities. Suitable oligomeric epoxy compounds containing a hydroxy functionality or (OH) group used in the practice of the present invention include, among others, sorbitol polyglycidyl ether such as DCE 358 (form Dixie Chemical Co., Houston, Tex.), diglycerol polyglycidy ether such as Denacol EX 421 (from Nagase Cehmical Co., Hyogo, Japan), glycerol polyglycidyl ether such as Denacol Ex 313 and EX 314 (from Nagase Chemical Co., Hyogo, Japan), and triglycidyl tris(2-hydroxyethyl) isocyanurate such as Denacol EX 421 (from Nagase Chemical Co., Hyogo, Japan). Suitable oligomeric epoxy compounds which typically do not contain significant hydroxy functionality used in the practice of the present invention include, among others, di- and polyglycidyl ethers of polycarboxylic acid and di- and polyglycidyl ester of acids such as Araldite CY 184 (from Ciba-Geigy, Brewster, N.Y.), cycloaliphatic epoxide such as ERL 4221 (from Union Carbide, Danbury, Conn.), and polyglycol diepoxide such as DER 736 (from Dow Chemical Co., Midland, Mich.). A composition of the present invention comprises from about 10 weight percent to about 40 weight percent of an epoxy compound, preferably from about 15 weight percent to 35 weight percent of an epoxy compound.
The coating composition preferably includes one or more catalysts to enhance crosslinking of the components on curing.
A preferable catalyst is an organotin catalyst such as dibutyltin dilaurate, dibutyltin diacetate, dibutyltin oxide, and dibutyltin bis(acetoacetate) in an amount of generally ranges from 0.001 percent to 1.0 percent, preferably from 0.01 percent to 0.5 percent and more preferably from 0.05 percent to 0.2 percent, the percentages being in weight percentages based on the total weight of composition solids.
In addition, the above coating composition can include other suitable catalysts, such as conventional acid catalysts, for example blocked or unblocked phosphoric acid and sulfonic acid in an amount ranging from 0.1 percent to 2 percent, preferably in the range of from 0.2 percent to 1.0 percent. As examples of such acid catalysts, phenyl acid phosphate, butyl acid phosphate, octyl acid phosphate, dodecylbenzenesulfonic acid, para-toluenesulfonic acid and dinonylnaphthalene sulfonic acid, which are optionally blocked with amines such as dimethyl oxazolidine and 2-amino-2-methyl-1-propanol can be mentioned.
The coating composition of the present invention, which is formulated into high solids coating systems may also contain at least one organic solvent typically selected from the group consisting of aromatic hydrocarbons, such as, petroleum naphtha or xylenes; ketones, such as, methyl amyl ketone, methyl isobutyl ketone, methyl ethyl ketone or acetone; esters, such as, butyl acetate or hexyl acetate; and glycol ether esters, such as propylene glycol monomethyl ether acetate. The amount of organic solvent added depends upon the desired solids level as well as the desired amount of VOC of the composition. If desired, the organic solvent may be added to both components of the binder. The amount of organic solvent used results in the composition having a VOC of less than 0.48 kilogram/liter (4 pounds per gallon), preferably in the range of 0.1 kilogram/liter to 0.4 kilogram/liter (1 pound to 3 pounds per gallon) of an organic solvent per liter of the composition.
The coating composition of the present invention may also contain conventional additives, such as, stabilizers, and rheology control agents, flow agents, and toughening agents. Such additional additives will, of course, depend on the intended use of the coating composition. Any additives that would adversely effect the clarity of the cured coating will not be included as the composition is used as a clear coating. The foregoing additives may be added to either component or both, depending upon the intended use of the coating composition.
To improve weatherability of the clear finish of the coating composition, about 0.1-5%, by weight, based on the weight of the composition solids, of an ultraviolet light stabilizer or a combination of ultraviolet light stabilizers may be added. These stabilizers include ultraviolet light absorbers, screeners, quenchers and specific hindered amine light stabilizers. Also, about 0.1-5% by weight, based on the weight of the composition solids, of an antioxidant can be added. Typical ultraviolet light stabilizers that are useful include benzophenones, such as hydroxydodecyclbenzo-phenone, 2,4-dihydroxybenzophenone; triazoles, such as 2-phenyl4-(2′-4′-dihydroxybenzoyi)triazoles; and triazines, such as 3,5-dialkyl-4-hydroxyphenyl derivatives of triazine and triazoles such as 2-(benzotriazole-2-yl)-4,6-bis(methylethyl-1-phenyl ethyl)phenol, 2-(3-hydroxy-3,5′-di-tert amyl phenyl) benzotriazole, 2-(3′,5′-bis(1,1-dimethylpropyl)-2′-hydroxyphenyl)-2H-benzotriazole, benzenepropanoic acid, 3-(2H-benzotriazol-2-yl)-5-(1,1-dimethylethyl)4-hydroxy-,C7-9-branched alkyl esters, and 2-(3′,5′-bis(1-methyl-1-phenylethyl)-2′-hydroxyphenyl)benzotriazole.
Typical hindered amine light stabilizers are bis(2,2,6,6-tetramethylpiperidinyl)sebacate, bis(N-methyl-2,2,6,6-tetramethylpiperidinyl)sebacate and bis(N-octyloxy-2,2,6,6-tetramethylpiperidynyl)sebacate. Particularly useful blend of ultraviolet light absorbers and hindered amine light stabilizers is bis(N-octyloxy-2,2,6,6-tetramethylpiperidynyl)sebacate and benzenepropanoic acid, 3-(2H-benzotriazol-2-yl)-5(1,1-dimethylethyl)4hydroxy-,C7-9-branched alkyl esters.
The coating composition of the present invention optionally contains, in the range of 0.5 weight percent to 15 weight percent, preferably 1 weight percent to 10 weight percent of polyhydroxy functional compounds such as polycarbonate polyol (formula shown in the example), 2-ethyl-1,3-hexanediol, polycaprolactone triol, and the adduct of epsilon caprolactone and 1,3,5-tris(2-hydroxyethyl) cyanuric acid, all percentages being based on the total weight composition solids.
The coating composition of the present invention optionally contains in the range of from 0.5 percent to 30 percent, preferably in the range of from 1 percent to 20 percent, stabilized crosslinked polymer particles, i.e., non-aqueous dispersion (NAD) (as described in detail in U.S. Pat. No. 4,960,828), all percentages being based on the total weight of composition solids.
The clear coating composition of the present invention may be supplied in the form of a two-pack coating composition in which a first pack includes the polyisocyanate and a second-pack includes the melamine, epoxy compounds and other additives.
Generally, the first and the second pack are stored in separate containers and mixed before use. The containers are preferrably sealed air tight to prevent reactions during storage.
Alternatively, when the isocyanate functionalities of the polyisocyanate are blocked, all components of the composition can be stored in the same containers in the form of a one-pack coating composition.
In use, the first-pack of the two-pack coating composition containing the polyisocyanate and the second-pack containing the melamine, epoxy compounds, and other additiaves are mixed just prior to use for about 1 to 15 minutes before use to form a pot mix, which has limited pot life, in range of from 5 minutes to 6 hours, before it becomes too viscous to permit application through conventional application systems, such as spraying. Alternatively, the first pack and the second pack are mixed together at the plural spray gun with or without electrostastics, before applied to a substrate. Other methods of applications include roller coating, dipping or brushing and other conventional application methods. The layer of the coating composition then cures under ambient conditions or preferably at higher temperatures in the range of 80-160° C. for 10 minutes to 3 hours, preferably in the range of 20 minutes to 1 hour to form a coating on the substrate having the desired coating properties.
When the one-pack coating composition containing the blocked polyisocyanate is used, a layer thereof applied over a substrate using aforedescribed application techniques, is cured at a baking temperature in the range of from 80° C. to 200° C., preferably in the range of 80° C. to 160° C., for about 10 to 60 minutes It is understood that actual baking temperature would vary depending upon the catalyst and the amount thereof, thickness of the layer being cured, the blocked isocyanate functionalities and the melamine utilized in the coating composition. The use of the foregoing baking step is particularly useful under Original Equipment Manufature (OEM) condition.
It is understood that the actual curing time depends upon catalyst type and amount, the thickness of the applied layer and on the presence or absence of any suitable drying devices, such as, fans that assist in continuously flowing air over the coated substrate to accelerate the cure rate. Generally, a clearcoat layer having a thickness in the range of from 25 micrometers to 75 micrometers applied over a metal substrate, such as automotive body which is often precoated with other coating layers such as electrocoat, primer and basecoat; cures in 20 to 60 minutes at at about 80° C. to 160° C.
Testing Procedures
The following test procedures were used for generating data reported in the examples below:
  Test   Test MethodDry film thickness   ASTM D1400AppearanceASTM D523, VISUAL20° GlossASTM D523DOIASTM D5767Tukon HardnessASTM D1474Mar resistanceASTM D5178Percent solidsASTM D2369